Photoinitiators for light-curable compositions

ABSTRACT

Compounds of formula (I) are photoinitiators or photosensitizers in a photopolymerizable composition: 
                         
R 1  represents a monovalent, linear, branched or cyclic, aliphatic hydrocarbon group having 1 to 20 carbon atoms, optionally substituted with substituent(s) selected from —Cl, —Br, —OH, ═O, —NH—CO—OR 2 , —NH—CO—R 2  or free-radically or ionically polymerizable groups. Each R 2  is independently —H or C 1-6  alkyl; n is ≥1. If n=1, Z and Y are absent and X represents —OR 3 ; if n is &gt;1, Z represents —OR 4 —, Y represents —OR 5 — and X represents —H or —OH. R 3  represents —H or R 1 ; and R 4  and R 5  each independently represent a bivalent hydrocarbon group. The polymerizable moieties as optional substituents of R 1  are polymerizable double or triple bonds, lactam, lactone and epoxide moieties, which are subjectable to ring-opening polymerization; and two of R 1  to R 5  may be linked to one another to form a ring or a dimer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of co-pending U.S. patent applicationSer. No. 16/633,385, filed Jan. 23, 2020, which is a Section 371 ofInternational Application No. PCT/EP2018/070464, filed Jul. 27, 2018,which was published in the German language on Jan. 31, 2019 underInternational Publication No. WO 2019/020805 A1, which claims priorityunder 35 U.S.C. § 119(b) to Austrian Application No. A 313/2017, filedJul. 27, 2017, the disclosures of which are incorporated herein byreference in their entirety.

The present invention relates to new photoinitiators for photocurablecompositions.

STATE OF THE ART

In the field of photopolymerization, numerous initiators and sensitizersare known to trigger polymerization reactions, the effect of type Iinitiators being based on the cleavage of an intramolecular bond, whilethat of type II initiators is based on the abstraction andintermolecular transfer of a hydrogen atom from a co-initiator to theinitiator molecule.

From the compound class of ketones, benzophenone and acetophenone aswell as various derivatives thereof have been known as initiators for along time, these include α-hydroxyalkyl phenones such as2-hydroxy-2-methylpropiophenone, as disclosed in DE 28 08 459 A1, whichused to be marketed by Ciby Geigy and is now marketed by IGM Resinsunder the name Darocur® 1173 and has been one of the most usedinitiators ever since the 1980s.

Further initiators based on acyl and, more specifically, benzoyl groupsinclude, for example, bisacyl derivatives of aryl phosphine oxides, asdisclosed in U.S. Pat. Nos. 4,737,593 A and 4,792,632 A, and bisbenzoylderivatives of alkyl, cycloalkyl and aryl phosphine oxides of thefollowing formula, as disclosed in EP 615,980 A2:

wherein R₁ may be a defined alkyl, cycloalkyl or aryl radical.

Moreover, glyoxylates having aromatic groups have been successfully usedas photoinitiators since the middle of the 1970s. See, for example, U.S.Pat. No. 4,038,164 A, wherein initiators of the following formula aredisclosed:

wherein R represents linear or branched-chain C₁₋₁₀ alkyl, aryl, aralkylor a mono-, di- or trialkylsilyl group; and R′ represents a heterocyclicradical, C₆₋₁₄ aryl or an optionally mono- or polysubstituted phenyl.

Recently, a new group of photoinitiators has become known from WO2017/060459 A1 and WO 2017/060527 A1 of Dentsply Detrey GmbH; they arereferred to as “compounds having an acylsilyl or acylgermyl group”,although they should, in summary, be rather referred to as silyl- orgermyl-substituted α-diketones or α-ketoesters or -amides and can berepresented by the following formula:

wherein X represents R₁R₂R₃Si— or R₁R₂R₃Ge—, wherein R₁ to R₃ eachindependently represent an optionally substituted hydrocarbon group, sothat X represents a trihydrocarbylsilyl or -germyl group; and

-   -   Y either represents one of the same options as mentioned for X,        wherein the hydrocarbyl groups may be different from those in X,        or represents —Z—R₄, wherein Z represents a chemical bond, —O—        or —NR′—, wherein R′ is an optionally substituted hydrocarbyl        radical, and R₄ represents an optionally substituted hydrocarbyl        radical or a trihydrocarbylsilyl group, wherein one or two of        the hydrocarbyl radicals on the silicon atom may also be        hydrocarbylcarbonyl radicals.

In any case, it is essential that the silicon or germanium atom beingsubstituted with three (optionally further substituted) hydrocarbonradicals is in the α position to the keto group of the α-diketone or theα-ketoester or -amide. As preferred embodiments of the silicon- and/orgermanium-containing initiators, various acetophenone derivativeswherein the α carbon atom has been replaced by Si or Ge arepredominantly disclosed.

Such photoinitiators are said to be particularly suitable, among otherpurposes, for curing dental material in a patient's mouth, as they showa high quantum yield of light absorption and are said not to migrate outof the uncured mass. Moreover, acid resis-tance, good solubility in thepolymerizable composition, thermostability and storage stability aredemanded.

According to WO 2017/060459 A1 and WO 2017/060527 A1, such initiatorsare preferably or even obligatorily, respectively, used in combinationwith a co-initiator; as such are disclosed electron donors, e.g. amines,in the first case and particular acrylates in the second case. The thusachieved advantages include a high polymerization efficiency and curingspeed and no discoloration during curing.

A disadvantage of these initiators, however, is that they are allsensitive to visible light. Moreover, the use of these photoinitiatorshaving aromatic groups in the areas of medicine and food is restricted.

Against this backdrop, the aim of the present invention was to providealternative photoinitiators based on α-ketoesters that are able to solveat least some of the above problems.

DISCLOSURE OF THE INVENTION

The present invention achieves the above aim by providing a novel use ofcompounds—more specifically α-ketoesters—of the following formula (I) asphotoinitiators or photosensitizers in a photopolymerizable composition:

wherein R₁ represents a monovalent, linear, branched or cyclic,aliphatic hydrocarbon group having 1 to 20 carbon atoms, which isoptionally substituted with one or more substituents selected from —Cl,—Br, —OH, ═O, —NH—CO—OR₂, —NH—CO—R₂ or free-radically or ionicallypolymerizable groups, wherein each R₂ radical is independently selectedfrom —H or C₁₋₆ alkyl;

-   -   n is ≥1, wherein        -   i) if n=1, Z and Y are not present and X represents —OR₃;            and        -   ii) if n is >1, Z represents —OR₄—, Y represents —OR₅— and X            represents —H or —OH; wherein            -   R₃ represents —H or one of the options mentioned for R₁;                and            -   R₄ and R₅ each independently represent a bivalent                hydrocarbon group for which otherwise the same options                as mentioned for R₁ apply;    -   wherein the polymerizable moieties as optional substituents of        R₁ are selected from polymerizable double or triple bonds as        well as lactam, lactone and epoxide moieties, which are        subjectable to ring-opening polymerization;    -   and wherein any two of R₁ to R₅ may be optionally linked to one        another to form a ring or a dimer.

When using the above compounds of formula (I) as photoinitiatorsaccording to the invention, they are usually at least equivalent toknown initiators, both to standard initiators such as benzophenone andto relatively new compounds according to WO 2017/060459 A1 and WO2017/060527 A1, in many cases even exceeding them, though.

Due to the lack of aromatic radicals and adjacent Si or Ge atoms, theα-ketoesters used according to the invention are physiologicallyharmless and do not tend to dis-integrate when exposed to visible light.The compounds of formula (I) are thus ideally suited for use asphotoinitiators and photosensitizers, as clearly demonstrated by theExamples below.

In preferred embodiments of the present invention, R₁ represents alinear, branched or cyclic aliphatic hydrocarbon group having 1 to 20carbon atoms, in which one or more carbon atoms may have been replacedby oxygen atoms and which is optionally substituted with one or moresubstituents selected from —Cl, —Br, —OH and —SH; and/or n is 1 and Xrepresents —OR₃, wherein R₃ independently represents —H or one of theoptions mentioned for R₁.

“A dimer of a compound of formula (I)” designates a compound that isformed by a formal coupling of any of the radicals R₁ to R₄ of acompound of formula (I) to any of the radicals R₁ to R₄ of an identicalor a different compound of formula (I). There are, for example, threeoptions when compounds of formula (I) in which n is 1 and X is —OR₃ arecoupled at their terminals: via the two radicals R₁, via one radical R₃of each of the two moieties X or via one radical R₁ and one radical R₃of a moiety X, as represented below:

In an essentially analogous manner, two molecules of formula (I) whereinat least one of the radicals R₁ and R₃ (if n is 1) or R₄ (if n is >1)comprises a C₁₋₆ alkyl group as the radical R₂, i.e., if —NR₂— as asubstitute for α carbon atom or a substituent —NH—CO—OR₂ or —NH—CO—R₂ ispresent, may be coupled via these radicals R₂— be it that either tworadicals R₂ are coupled to one another or one radical R₂ is coupled to aR₁, R₃ or R₄ radical of the other molecule. Couplings between one R₁radical and one R₃ or R₄ radical or between two R₃ or R₄ radicals arealso included in the scope of the invention.

This applies in a similar manner for an optional intramolecularcyclization between any two of the radicals R₁ to R₃ (if n is 1) or R₄(if n is >1) of the same molecule. Below, a terminal coupling for a casewhere n is 1 between R₁ and the radical R₃ (or a radical R₂ includedtherein) of the moiety X via an intermediate pentylene group that can beconsidered as a part of either radical R₁ or radical R₃ (or R₂) of themoiety X is illustrated as an example.

In addition to the above described preferred embodiments of theα-ketoesters of the invention, wherein n is 1, Z and Y are not presentand X represents —OR₃, so that R₁ and X are each directly bound to oneof the two C═O moieties, special embodiments wherein n is >1, Zrepresents —OR₄, Y represents —OR₅— and X represents —H or —OH are alsopreferred. As a result, these are oligomers or polymers of glyoxylicacid esters wherein the photoactive glyoxylic acid motif is contained ntimes. Particularly preferred embodiments are those wherein the carbonatom of radical R₄ of the moiety Z, which is adjacent to the oxygen atomis substituted with an oxo group ═O, which results in a carboxyl group,and wherein X represents —OH. Thus, such molecules are polyesters whichare comparably easily accessible via a polycondensation ofα-ketodicarboxylic acids with diols, which is why embodiments wherein R₁comprises a terminal OH group, as generally represented below, areparticularly preferred:

Therein, HO—R₆— represents the radical R₁— and —(C═O)—R₇— represents theradical —R₄—in formula (I), while X is —OH. The radicals R₄ and R₇ thusdefine the dicarboxylic acid, while the radical R₅ defines the diol fromwhich such polymers are obtainable. If radical R₇ is —(CH₂)₂—, forexample, this will result in α-ketoglutaric acid as the startingmaterial for the polycondensation reaction while, if both radicals R₅and R₆ are —(CH₂)₆—, hexanediol will be the starting material, as thiswill be disclosed in the Examples later on.

The value of n defines the chain length of the compounds of formula (I)and is generally not particularly limited as long as the products areeffective as photoinitiators or photosensitizers in the inventivemanner. As a plurality of free radical centers are formed at the sametime when such polymers of formula (I) are irradiated, suchphotoinitiators or photosensitizers provide for particularly rapidpolymerization reactions. For the reason of an easier handling,according to the present invention, the value of n is preferably limitedto 100, more preferably to 50, In some cases, significantly lowervalues, e.g., ranging from 2 to 10, may be preferred, too.

Those skilled in the art will understand that the compounds of formula(I) may not only be linked to one another to form dimers, but may alsobe bound to other molecules. Terminal OH groups, as contained in theabove-described preferred embodiments of the invention that usepolyesters of formula (I), are particularly easily accessible instandard derivatization reactions and may, for example, be bound easilyto pre-polymers and other polymer elements in the formulations in whichthey are to be used as initiators or sensitizers or may also beimmobilized onto solid phases. Forming alcoho-lates, preferably withalkali metal cations such as Li⁺, Na⁺ or K⁺, may also prove advantageousin certain circumstances. Moreover, it is clear that some of the oxygenatoms contained in the compounds of formula (I) may be replaced bysulfur atoms, as long as this does not result in a loss of themolecules' reactivity as photoinitiators or photosensitizers.

According to the present invention, the compound of formula (I) may beused as both type I photoinitiator and type II photoinitiator, i.e.,depending on the reaction conditions, an intramolecular bond may becleaved or a hydrogen atom may be abstracted from a solvent orco-initiator molecule and transferred to the initiator of formula (I),as demonstrated later on in the exemplary embodiments.

For this reason, in preferred embodiments of the present invention,particularly in those where it serves as a type II photoinitiator, thecompound of formula (I) is used in combination with one or moreco-initiators in the photopolymerizable compositions.

As the co-initiators, one or more compounds selected from mono- orpolyhydric alcohols (—OH), thiols (—SH), amines (—NR—), silanes (═SiH—),germanes (═GeH—), phosphines (—PRR′R″—), ethers (>CH—O—CH<), iodonium(—I⁺—) and sulfonium (═S⁺—) salts and derivatives thereof are preferablyused, since such compounds proved their worth as co-initiators in thepast.

Even more preferably, one or more compounds selected from sugar basedpolyols, glycerol or the like, thiols, polyethylene glycol orpolypropylene glycol are used as co-initiators, glycerol being mostpreferably used as a polyhydric alcohol, which proved particularlyuseful as a co-initiator for compounds of formula (I) in the exemplaryembodiments.

The compound of formula (I) is preferably used in an amount of 0.1 to 10parts by weight, more preferably 0.5 to 5 parts by weight, mostpreferably approx. 1 to 2 parts by weight, per 100 parts by weight ofpolymerizable monomers in the composition.

Additionally, the compound of formula (I) is preferably used for curingfree-radically polymerizable monomers such as acrylates, methacrylates,maleinimides, styrene derivatives, vinyl esters, vinyl carbonates, vinylcarbamates, and the like, as well as ring-opening monomers such as vinylcyclopropanes, vinyl cyclooxiranes, vinyl cyclo-butanes, ketene acetals,vinyl spiroesters, and the like, even more preferably for curingacrylate and methacrylate monomers, since they have proved clearlysuperior to standard initiators in such compositions.

As they are physiologically harmless, the compounds of formula (I) mayadditionally be preferably used as initiators in compositions to becured inside the human body or for curable compositions in the foodsector. This distinguishes them from the majority of standardinitiators.

Further components of the polymerizable compositions are notparticularly limited as long as they do not have any negative impact onthe initiators' efficiency and the curing process. Any suitable fillers,solvents, further initiators or sensitizers, plasticizers, flowenhancers and the like may thus also be used, for example.

A second aspect of the present invention thus relates to aphotopolymerizable composition which is characterized in that itcomprises at least one compound of formula (I) as a photoinitiator orphotosensitizer, at least one photopolymerizable monomer, optionally atleast one co-initiator, and optionally further components as definedabove.

EXAMPLES

The present invention will now be described in more detail withreference to represen-tative examples that only serve the purpose ofillustrating the invention without limiting its scope.

Materials and Procedures

If not stated otherwise below, all reagents and photoinitiators wereobtained from commercial sources and used without further purification.¹H NMR and ¹³C NMR spectra were mostly recorded using a Bruker DPX-200Fourier Transform spectrometer at 200 MHz or 50 MHz, respectively, andsome measurements were carried out using a BrukerAvance at 400 MHz (¹H)and 100 MHz (¹³C). Mass spectra were recorded using a Thermo FisherScientific ITQ 1100 and a silica capillary column (30 m×0.25 mm).

The following photoinitiators of formula (I) were tested in the Examplesof the present invention:

2-Oxopropanoic acid ethyl ester (pyruvic acid ethyl ester, ethylpyruvate) (1):

2-Oxobutanoic acid methyl ester (2-oxobutyric acid methyl ester) (2):

2-Oxo-3-methylbutanoic acid ethyl ester (2-oxoisovaleric acid ethylester) (3):

2-Oxo-3,3-dimethylbutanoic acid methyl ester (4):

3-Bromo-3-methyl-2-oxobutanoic acid ethyl ester (5):

3-Hydroxy-3-methyl-2-oxobutanoic acid ethyl ester (6):

N,N′-Dimethylaminopyruvate (7)

4,4-Dimethyldihydrofuran-2,3-dione (8):

α-Ketoglutaric acid diethyl ester (9):

N,N′-Dimethylaminopyruvate (10)

α-Ketoglutaric acid di(hydroxyethylmethacrylate) ester (11):

α-Ketoglutaric acid-hexanediol polyester (Mn ˜10000 Da) (12)

Urethane methacrylate-terminated α-ketoglutaric acid-hexanediolpolyester (13)

The initiators (1) to (4) were obtained from commercial sources, whilethe novel initiators (5) to (13) were synthesized by the inventors, asdescribed in more detail in the Synthesis Examples later on.

In the Comparative Examples, the following four known, commerciallyavailable initiators were tested:

Benzophenone (BP):

Ethyl phenylglyoxylate (PGO)

2-Bromo-2-methyl-1-phenylpropan-1-one (2-bromoisobutyrophenone) (“bromodarocure”, BD):

3-(4-Benzoylphenoxy)-2-hydroxy-N,N-dimethylpropane-1-amine hydrochloride(BPQ)

Synthesis Example 1 Synthesis of 3-bromo-3-methyl-2-oxobutanoic acidethyl ester (5)

Into a 50 ml three-necked flask, 15 ml of CCl₄ were placed. Thereto, 1equivalent (5.62 g, 39 mmol) of 3-methyl-2-oxobutanoic acid ethyl esterwas added, followed by 1 eq. (6.24 g, 39 mmol) of bromine. Then, 1.5 mlof acetic acid was used for acidifica-tion. After decoloring thesolution, the reaction was quenched using approx. 30 ml of sat. NaHCO₃solution, and the mixture was transferred into a separating funnel.There, 100 ml diethyl ether and another 20 ml of NaHCO₃ were added. Theaqueous layer was discarded, and the organic layer was re-extracted with50 ml of NaHCO₃. The ether layer was washed with sat. NaCl solution anddried over sodium sulfate. The solvent was then removed on a rotaryevaporator.

Yield: 6.94 g (79% of theory)

¹H NMR (400 MHz, C₆D₆) δ ppm: 4.31 (q, 2H, J=7.02 Hz); 1.95 (s, 6H);1.32 (t, 3H, J=7.2 Hz).

GC-MS: 224.99 [M+H]⁺, 143.07 [M−Br]⁺, 122.93 [M−COCOOEt]⁺, 70.20[M−Br-COOEt]⁺.

Synthesis Example 2 Synthesis of 3-hydroxy-3-methyl-2-oxobutanoic acidethyl ester (6)

1 equivalent (4.46 g, 20 mmol) of 3-bromo-3-methyl-2-oxobutanoic acidethyl ester was added to 0.5 eq. silver(I) oxide in 40 ml moistacetonitrile (ACN). The mixture was stirred for 12 hrs, and thelight-grey precipitate was removed by filtration. The solution was thendiluted using 100 ml of water and extracted with 200 ml of ether, andthe organic layer was dried over sodium sulfate. After removing thesolvent, the crude product was subjected to fractional distillation(120° C., 25 mbar).

Yield: 1.50 g (93% of theory)

¹H NMR (400 MHz, C₆D₆) δ ppm: 3.85 (q, 2H, J=7.31 Hz); 2.94 (bs, 1H);1.30 (s, 6H); 0.83 (t, 3H, J=7.09 Hz).

GC-MS: 161.16 [M+H]⁺, 143.20 [M−OH]⁺, 115.16 [M−OEt]⁺, 87.09 [M−COOEt]⁺,59.06 [(CH₃)₂OH]⁺.

Synthesis Example 3 Synthesis of N,N′-dimethylamino pyruvate (10)

In a 100 ml three-necked round-bottom flask, 2,5 eq. N,N-dimethylethanolamine were added dropwise from a septum to a mixture of 1 eq.3,3-dimethyl-2-oxobutanoyl chloride in 10 ml dichloromethane whilecooling using an ice bath. After the addition had been completed,stirring was continued for 48 hrs at room temperature. After thereaction had been completed, the solvent was removed using a rotaryevaporator. The product was obtained as a clear oil by Kugelrohrdistillation at 1.2 mbar and 94° C.

Yield: 177 mg (17% of theory)

¹H NMR (400 MHz, C₆D₆, ppm): 4.11 (t, 2H), 2.29 (t, 2H), 2.02 (s, 6H),1.16 (s, 9H).

2.5 eq. N,N-dimethylethanolamine were added dropwise from a septum to amixture of 1 eq. 3,3-dimethyl-2-oxobutanoyl chloride in 10 mldichloromethane. After the addition had been completed, stirring wascontinued for 48 h at room temperature. After the reaction had beencompleted, the solvent was removed using a rotary evaporator. Theproduct was obtained as a clear oil by Kugelrohr distillation at 1.2mbar and 94° C.

Yield 177 mg (17% of theory)

¹H NMR (400 MHz, C₆D₆, ppm): 4.11 (t, 2H), 2.29 (t, 2H), 2.02 (s, 6H),1.16 (s, 9H).

Synthesis Example 4 Synthesis of α-ketoglutaric acid diethyl ester (9)

At first, α-ketoglutaric acid was re-crystallized from acetone.Subsequently, 1 eq. (3.662 g, 25 mmol) of α-ketoglutaric acid wereweighed into a 250 ml three-necked round-bottom flask equipped with acondenser and a septum, followed by the addition of 100 ml of ethanoland 0.3 eq. (0.4 ml, 7.5 mmol) of sulfuric acid. The reaction mixturewas magnetically stirred and refluxed. The oil bath was set to 100° C.,and the reaction was left to proceed for 24 h. Reaction progress wasmonitors by thin-layer chromato-graphy (PE:EE, 5:1; Rf 0.36). Thesolvent was evaporated using a rotary evaporator, followed by theaddition of 50 ml of de-ionized water and neutralization of the solutionusing 1N KOH solution. The aqueous layer was extracted three times with100 ml of ehtylacetate, and the combined organic layers were dried overNa₂SO₄. The solvent was then evaporated using a rotary evaporator. 4.193g (83% of theory) crude yield were obtained. The crude product waschromatographically separated by means of MPLC using a 214 g silicacolumn (PE:EE, 9:1). The product was detected using a UV detector (254nm) and identified by DC (PE:EE, 5:1; Rf 0.36). The fractions containingthe product were combined, and the solvent was evaporated using a rotaryevaporator. In total, 1.77 g (35% of theory) of α-ketoglutaric aciddiethyl esther (9) were obtained as a colorless oil.

Rf: 0.36 (PE:EE, 5:1)

¹H NMR (400 MHz, CDCl₃, ppm): 4.32 (q, J=7.2 Hz, 2H), 4.13 (q, J=7.20Hz, 2H), 3.14 (t, J=6.6 Hz, 2H), 2.65 (t, J=6.6 Hz, 2H), 1.36 (t, J=7Hz, 3H), 1.24 (t, J=7 Hz, 3H).

¹³C NMR (400 MHz, CDCl₃, ppm): 192.8, 172.0, 160.6, 62.6, 60.9, 34.2,27.8, 14.2, 14.0.

GC-MS: 202.99 [M+H], 129.94 [M+H, —(CO—O—CH₂—CH₃)], 128.95[M−(CO—O—CH₂—CH₃)], 101.99 [M+H, —(CO—CO—O—CH₂—CH₃)], 101.00[M−CO—CO—O—CH₂—CH₃], 74.11 [M+H, —(CO—CH₂—CH₂—CO—O—CH₂—CH₃)], 73.14[M−(CO—CH₂—CH₂—CO—O—CH₂—CH₃)], 55.13 [M+H, —(O—CH₂—CH₃),—(CH₂—CH₂—COO—CH₂—CH₃)].

Synthesis Example 5 Synthesis of α-ketoglutaric aciddi-2-hydroxyethylmethacrylic acid ester (11)

At first, α-ketoglutaric acid was re-crystallized from acetone.Subsequently, 1 eq. (3.943 g, 27 mmol) of α-ketoglutaric acid, 2 eq.(7.290 g, 54 mmol) of 2-hydroxymethyl methacrylate, 0.7 wt % (78.5 mg)lipase (Candida antarctica) immobilized on acrylic resin (<5.000 U/g)and 565 ppm (10.1 mg) of 2,6-di-tert-butyl-4-methylphenol (BHT) wereplaced in a 50 ml single-necked round-bottom flask. The flask wasequipped with a drying tube filled with calcium chloride. The mixturewas magnetically stirred in an oil bath at 65° C. Reaction progress wasmonitored by NMR and TLC (PE:EE, 8:2; Rf 0.62), and after a reactiontime of 140 hrs, the reaction was diluted with PE:EE (1:1) andchromatographically separated using a 90 g silica column by MPLC (PE:EE,8:2). The combined product fractions were treated with BHT andevaporated using a rotary evaporator. 2.991 g (30% of theory) of thepure product were obtained as a colorless, transparent oil.

Rf: 0.62 (PE:EE, 8:2)

¹H NMR (400 MHz, (acetone-d₆, ppm): 6.05 (s, 2H, 2 CHH), 5.62-5.59 (m,2H), 4.54-4.48 (m, 2H), 4.43-4.38 (m, 2H), 4.30 (s, 4H), 3.15 (t, J=6.6Hz, 2H), 2.63 (t, J=6.6 Hz, 2H), 1.87 (t, J=1.2 Hz, 6H).

Synthesis Example 6 Synthesis of ketoglutaric acid-hexanediol polyester(12)

At first, α-ketoglutaric acid was re-crystallized from acetone andp-toluene sulfonic acid was re-crystallized from chloroform.Subsequently, 1 eq. (11.18 g, 80 mmol) of pure α-ketoglutaric acid, 1.01eq. (9.54 g, 81 mmol) 1,6-hexanediol and 0.0025 eq. (30.2 mg, 0.2 mmol)of pure p-toluene sulfonic acid were weighed into a 100 ml three-neckedflask equipped with a magnetic stirrer and a Dean Stark trap. 30 ml abs.toluene were added, and the oil bath was heated to 125° C. Reactionprogress was monitored by NMR, and the reaction was stopped after 24hrs. The dissolved polyester was diluted with 30 ml abs. toluene andprecipitated in 800 ml cold diethyl ether. The result obtained was aslightly yellow polyester that was dried in a vacuum drying oven at 50°C., followed by dissolution in 60 ml THF, filtering the solution andre-precipitation in 800 ml cold diethyl ether. After drying undervacuum, 11.36 g (62%) of the polyester were obtained as a white polymer.

Molecular weight, Mn (by GPC and NMR)≈10,000; n≈40

¹H NMR (400 MHz, CDCl₃, ppm): 4.26 (t, J_(HH)=6.6 Hz, 40H), 4.08 (t,J_(HH)=6.6 Hz, 40H), 3.65 (t, J_(HH)=6.6 Hz, 4H), 3.15 (t, J_(HH)=6.6Hz, 40H), 2.67 (t, J_(HH)=6.6 Hz, 40H), 1.78-1.72 (m, 40H), 1.68-1.60(m, 40H), 1.45-1.36 (m, 80H).

Synthesis Example 7 Synthesis of α-ketoglutaric acid-1,6-hexanediolpolyester having 2-isocyanatoethyl-methacrylate-modified terminal groups(13)

In a first step, 1 eq. of the polyester (0.98 g, 0.2 mmol), 2 dropsdibutyltin dilaurate as a catalyst and 20 ml of abs. toluene werecharged into a 50 ml three-necked round-bottom flask. The flask wasflushed with argon and equipped with a septum and an argon balloon.Subsequently, 2.05 eq. (0.6 ml, 0.4 mmol) of2-isocyanatoethylmeth-acrylate were added dropwise via the septum. Themixture was stirred for 14 hrs at room temperature and then quenchedusing 5 ml of methanol. 20 ml of distilled acetone were added, and thepolyester was precipitated in 300 ml of cold diethyl ether, yielding awhite polymer (0.36 g, 34% of theory).

Molecular weight, Mn (by GPC and NMR)≈10,000; n≈40

¹H NMR (400 MHz, CDCl₃, ppm): 6.12 (s, 2H), 5.59 (s, 2H), 5.342 (s, 2H),4.26 (t, J_(HH)=6.6 Hz, 80H), 4.15 (t, 4H, J_(HH)=6.4 Hz), 4.08 (t,J_(HH)=6.6 Hz, 80H), 3.65 (t, J_(HH)=6.4 Hz, 4H), 3.15 (t, J_(HH)=6.6Hz, 80H), 2.67 (t, J_(HH)=6.6 Hz, 80H), 1.95 (s, 3H) 1.78-1.72 (m, 80H),1.68-1.60 (m, 80H), 1.45-1.36 (m, 160H).

In the following examples, of the inventive use of compounds of formula(I) as photoinitiators, reaction mixtures containing the respectivephotoinitiator, the specified liquid monomer and optionally a specifiedco-intiator were produced and cured for 10 min under a nitrogenatmosphere by exposure to an OmniCure® S2000 mercury lamp having a wavelength filter of 320 to 500 nm and an UV light intensity of 1 W/cm²,while the progress of the reactions was monitored by photo-DSC typeNETSCH DSC 204 F1 Phoenix.

All measurements were carried out at least twice, the respective tablesshowing the average values for the respective initiators.

In the tables, R_(P) represents the polymerization rate and is thus anindicator of the reactivity of a system. A high value means that manymonomer groups are reacted at the same time and that the curing processis generally shorter. t_(max) is the time (in s) it takes until maximumheat development is reached and is thus an indicator of how long ittakes to reach the gelling point and thus a certain initial solidity.Short times are thus more desirable. t_(95%) is the time (in s) afterwhich 95% of the entire reaction heat have been released and is thus anindicator of the rate at which a reaction occurs, lower values againbeing advantageous. DBC is the double bond conversion rate calculatedbased on the reaction heat released during polymerization (in J per g)of the respective formulation. For the conversion rate, it is desirableto achieve values that are as high as possible.

In all experiments, acrylates and methacrylates that are typically usedin the field of coating were used as monomers.

In all formulations, the respective photoinitiator was used in anequimolar amount with respect to 1 wt % of ethyl pyruvate (1), which wasthe initiator having the lowest molecular weight, and weighed inaccordingly, and co-initiators, if contained, were weighed in equimolaramounts with respect to the respective initiator. Subsequently, 12±0.5mg of the reaction mixtures were weighed into DSC aluminum pans, and thepans were covered with cover glasses.

Examples 1 to 5, Comparative Example 1—Type II Initiators

In this group of experiments, the hydrogen abstractors of the Examples 1to 4 (E1 to E4) and one Comparative Example (C1) were tested incombination with a co-initiator serving as a hydrogen donor.

Initiators:

Comparative Example 1

Monomer:

Co-Initiator:

4-dimethylaminobenzoic acid ethyl ester (DMAB)

Results:

TABLE 1 R_(P) t_(max) t_(95%) DBC [mmol · l⁻¹ · s⁻¹] [s] [s] [%] C1  9610.1 85.5 68.9 E1 230  7.7 41.7 66.5 E2 204  8.5 40.5 60.9 E3 241  7.536.6 65.8 E4 170 10.6 42.5 59.1 E5 218  6.0 73.0 63.1

Table 1 shows that the novel α-ketoesters according to the presentinvention achieve surprisingly high polymerization rates when comparedto benzophenone (BP), the industrial reference. This is mainly shown bythe fact that the time until 95% of the overall conversion is reached(t_(95%)) is significantly shorter. The final conversion (DBC) remainsat a level comparable to that using the industrial BP/DMAB system. Inparticular, the compounds of Examples 1 to 3 are distinguished by theirhigh reactivity and a particularly high conversion rate, among whichcompounds, methylethyloxobutanoate (3) of Example 3 achieved outstandingvalues.

Examples 6 and 7, Comparative Example 2—Type II Initiators

Two of the above experiments were repeated using the same two initiatorsand BP as a Comparative Example, using a non-aromatic co-initiator(MDEA) instead of the aromatic amine (DMAB).

Initiators:

Monomer:

Co-Initiator:

Results:

TABLE 2 R_(P) t_(max) t_(95%) DBC [mmol · l⁻¹ · s⁻¹] [s] [s] [%] C2 10910.8 77.0 69.3 E6 228  6.7 56.5 65.2 E7 234  7.7 41.0 64.6

It was also possible to achieve good results using the non-aromaticco-initiator with the compounds (3) and (4). The two ketoesters showed asignificantly higher reactivity and lower t_(95%) than the referencesystem.

Examples 8 and 9, Comparative Example 3—Type II Initiators

The compounds (1) and (4) were compared to the commercial initiatorethylphenyl glyoxylate (PGO).

Initiators:

Monomer:

Results:

TABLE 3 R_(P) t_(max) t_(95%) DBC [mmol · l⁻¹ · s⁻¹] [s] [s] [%] C3 20414.1 63.0 75.1 E8 210  8.5 56.0 69.3 E9 214  8.3 53.5 64.6

Surprisingly, the novel compounds (1) (Example 8) and (4) (Example 9)achieved significantly higher polymerization rates than the known phenylglyoxylate initiator (PGO) (Comparative Example 3). The maximumpolymerization rate was reached in almost half the time. The finalconversion was also achieved up to 10 s earlier when using the novelα-ketoesters.

Example 10—Type II Initiator

The compound that had achieved the best results in the experiments sofar, compound (3), was tested using thiol as a co-initiator; theachieved results were compared to those of Example 3 where DMAB had beenused as a co-initiator.

Initiator:

Monomer:

Co-Initiator:

Results:

TABLE 4 R_(P) t_(max) t_(95%) DBC [mmol · l⁻¹ · s⁻¹] [s] [s] [%] E3 241.0 7.5 36.6 65.8 E10 233.5 8.0 45.0 68.0

It can be seen that also the thiol is very well suited for use as aco-initiator for type II α-ketoester initiators.

Example 11, Comparative Example 4—Type I Initiators

In this experiment, the brominated ketoester (5) from Synthesis Example1 was compared to a known initiator that also contained bromide, “bromodarocur” (BD). As these two initiators were type I initiators,polymerization was carried out without any co-initiators.

Initiators:

Monomer:

Results:

TABLE 5 R_(P) t_(max) t_(95%) DBC [mmol · l⁻¹ · s⁻¹] [s] [s] [%] E11 2395.9 40.0 62.1 C4  146 7.3 62.5 57.6

The use of the novel α-ketoester (5) according to the invention providedclearly better results than the common bromide-containing type Iinitiator according to the state of the art, the novel compound (5)surprisingly even achieving significantly better reactivity results thanthe latter.

Examples 12 to 15, Comparative Example 5—Type II Initiators

In this group of experiments, the experiments from Examples 1 to 4 andComparative Example 1 were repeated, using a di-methacrylate mixturefrom the field of dental engineering as a monomer instead of thediacrylate that had been used above.

Initiators:

Monomers:

Co-Initiator:

Results:

TABLE 7 R_(P) t_(max) t_(95%) DBC [mmol · l⁻¹ · s⁻¹] [s] [s] [%] C5 21.4 18.5 146.5 50 E12 74.5 11.9  83.0 57 E13 71.2 12.2  85.5 57 E1481.5 11.0  58.5 57 E15 55.9 14.9  69.5 51

It becomes clear that the methacrylate monomer can also be cured moreefficiently using α-ketoesters as initiators than when usingbenzophenone. The reactivity of the ketoesters (1) to (4) of theinvention is surprisingly several times—up to 2.5 times—higher than thatof benzophenone, and the time until the peak maximum is reached is muchshorter than in the Comparative Example. In Examples 12 to 15 of theinvention, the double bond conversion was much higher using themethacrylate monomer than when using the reference initiator, andmaximum conversion was even reached 2 to 3 times faster, which furtherunderlines that the present invention is superior to the state of theart.

Example 16, Comparative Example 6—Type II Initiators

In this Example, a ketoester, compound (1), was directly compared bymeans of photo-DSC measurements to benzophenone (BP) as a known type IIinitiator when it comes to curing a known difunctional vinyl ester, i.e.divinyl adipate (DVA), that was used as a monomer.

Initiators:

Monomer:

Co-Initiator:

Results:

TABLE 8 DSC t_(max) Area [mW · mg⁻¹] [s] [J.g⁻¹] E16 1.7  194 466 C6 4.37  55 642

It becomes evident that, when using the vinyl ester monomer DVA that israther unreactive when compared to (meth)acrylates, the ketoester (1)according to the present invention achieves clearly worse results thanbenzophenone in the Comparative Example. This comparison, however, stillshows that it is generally possible to cure biocompatible monomers suchas DVA using food-safe α-ketoesters. As benzophenone and othercommercially available initiators may be harmful to health, theadvantage of their usability in this context will offset the lowerreactivity of vinyl esters.

Examples 17 and 18, Comparative Example 7—Type II Initiators inHydrogels

Using the partially water-soluble ketoesters in a hydrogel formulationwith PEG700DA (polyethylene glycol diacrylate with Mn 700) and 50 wt %water for polymerizing hydrogels worked surprisingly well. PBQ was usedas a commercially available reference initiator, with MDEA as aco-initiator. The ketoesters ethyl pyruvate (1) and dimethyl-furandione(8) can be used as novel initiators without any co-initiator. Due to itslong-wave absorption maximum (λ_(max)=378 nm instead of 330 nm forcompound (1)), compound (8) can also be used using visible light whichis harmless for cells.

Initiators:

Monomer:

Co-Initiator:

Results:

TABLE 9 R_(P) t_(max) t_(95%) DBC [mmol · l⁻¹ · s⁻¹] [s] [s] [%] C7   30 8.8 87 50 E17* 33  8.5 83 48 E18* 26 10.3 78 56 *without co-initiator

As becomes clear from Table 9, the biocompatible initiators of theinvention proved surprisingly reactive and achieved results that werecomparable to those achieved using the commercial initiator. In thisconnection, it has to be stated that in a study using the mousefibroblast cell line L929 and the commercial compound BPQ, significanttoxic effects were already observed at a concentration of 8 mmol/1,while the compounds (1) and (8) did not show any cytotoxicity even atconcentrations that were twice as high. This may constitute anadditional advantage for biological applications in which hydrogels arecommonly used.

Examples 19 and 20, Comparative Examples 8 and 9—Type II Initiators

Surprisingly, the biocompatible compound (8) from Example 18 was equallysoluble in aqueous and non-aqueous monomer systems. For this reason,reactivity was tested in acrylates (Example 19) and methacrylates(Example 20); in the latter case, DMAB was additionally used asco-initiator because of the low reactivity of the methacrylates. The newsystem was compared with the industrial standard initiator benzophenone(BP) in acrylates (Comparative Example 8) and methacrylates (ComparativeExample 9).

Initiators:

Monomers:

Co-Initiator:

Results:

TABLE 10 R_(P) t_(max) t_(95%) DBC [mmol · l⁻¹ · s⁻¹] [s] [s] [%] C8  93 11.1  69 69 C9   21 18.2 143 49 E19 189 11.6  58 66 E20  52 18.2  9054

As can be seen in table 10, the reactivity of compound (8) of theinvention without any co-initiator in HDDA (Example 19) was surprisinglymore than twice as high as that of the reference BP without co-initiator(Comparative Example 8). A comparable final conversion is also achievedmore rapidly. In methacrylates, the use of DMAB as a co-initiator(Example 20) of compound (8) yields not only a polymerization rate thatis more than twice as high, but, at the same time, also a significantlyhigher final conversion much more rapidly than in the reference example(Comparative Example 9).

Example 21 and 22, Comparative Example 10—Type II Initiators

Ketoglutaric acid was esterified with simple alcohols (Example 21) andhydroxyethyl methacrylate (HEMA) Example 22) to produce polymerizableinitiators, and their reactivity was tested in the di-acrylate HDDA.Reactivity was again compared to that of a benzophenone/amine system(Comparative Example 10).

Initiators:

Monomer:

Co-Initiator:

Results:

TABLE 11 R_(P) t_(max) t_(95%) DBC [mmol · l⁻¹ · s⁻¹] [s] [s] [%] C10 93 11.1 69 69 E21 201  9.2 58 67 E22 203 11.6 53 64

Surprisingly, the polymerization rate of these α-ketoesters was also atleast twice as high as that of the BP/amine reference system inComparative Example 10. A high final conversion is also achieved morerapidly. Additionally, the polymerizable groups of compound (11) makesure that any residual initiator will not be able to diffuse from thepolymer after polymerization, which is particularly important forapplications in the fields of medicine and food.

Examples 23 and 24 and Comparative Example 11—Type II Initiators

α-Ketoglutaric acid esters can also be used for methacrylates. For thisreason, substances (9) and (11) were tested in comparison withbenzophenone (BP) in the dental formulation DDM, using DMAB as aco-initiator.

Initiators:

Monomers:

Co-Initiator:

Results:

TABLE 12 R_(P) t_(max) t_(95%) DBC [mmol * l⁻¹ * s⁻¹] [s] [s] [%] C11 2118.2 143 49 E23 66 14.3  85 53 E24 57 14.5  93 52

Surprisingly, the polymerization rate achieved in unreactivemethacrylates using the novel α-ketoglutaric acid esters and DMAB as aco-initiator was 3 times as high as that of the reference system, as canbe seen from table 12. Polymerization proceeds much more rapidly than incase of the reference substance BP and results more rapidly in higherfinal conversion rates.

Example 25, Comparative Example 12—Polymeric Type II Initiators

As α-ketoglutaric acid is a dicarboxylic acid, it was also possible toproduce polyesters from hexanediol and α-ketoglutaric acid. Polymericinitiators have the advantage that they do not migrate from theresulting polymer, which is why they are used in the fields of food andmedicine. Additionally, polyesters can be modified in various ways, sothat their solubility, functionality, and degradability can be adaptedto specific applications.

Initiators:

Monomer:

Co-Initiator:

Results:

TABLE 13 R_(P) t_(max) t_(95%) DBC [mmol · l⁻¹ · s⁻¹] [s] [s] [%] C12 93 11.1 69 69 E25 144  9.2 71 64

As illustrated in table 13, the polyester (12) of Example 25 shows anunexpectedly high reactivity when compared to benzophenone (BP) inComparative Example 12. In spite of its high molecular weight and theassociated slow diffusion, the polymerization rate is more than onethird higher. α-Ketoglutaric acid polyesters are known for theirbio-compatibility and their biodegradability. The use of this class ofsubstances as photoinitiators is, however, completely new, and the highpolymerization rates were very surprising.

Example 26—Type II Initiator, Curing Test

Amines are used as co-initiators to increase the reactivity of type IIinitiators. It is also possible to bind the co-initiator covalently tothe initiator in order to avoid two-compo-nent systems. Compound (10) issuch an initiator that was tested using hexanediol diacrylate (HDDA) asa monomer in the present example.

Initiator:

Monomer:

1 g of a mixture of HDDA and 1 wt % of compound (10) were irradiated ina silicone mold placed in an Intelli-Ray 400 W UV oven at 100% power.After a few seconds, a transparent, hard sample rod was obtained,proving the suitability of the present invention for synthesizing moldedarticles.

Example 27—Polymeric Type II Initiator, Curing Test

Undesired migration of unreacted initiator molecules or theirdegradation products during irradiation constitutes a major problem, inparticular in the fields of food and medicine. This is whymacromolecular initiators and, in particular, macromolecular initiatorshaving polymerizable terminal groups are of great interest. Its highmolecular weight and reactive terminal groups prevent the initiator fromdiffusing out of the resulting polymer. In this connection, compound(13) was examined using hexanediol diacrylate (HDDA) as a monomer.

Initiator:

Monomer:

1 g of a mixture of HDDA and 1 wt % of compound (13) was exposed in asilicone mold in an Intelli-Ray 400 W UV oven at 100% power. After a fewseconds, a clear, hard sample rod was obtained, which proved that thepolymeric initiator according to the present invention was suitable forsynthesizing molded articles.

The above examples clearly show that, depending on the specific reactionconditions, the use of α-ketoesters provides a number of advantagescompared to known initiators, so that the present invention constitutesa valuable addition to the state of the art.

The invention claimed is:
 1. A photoinitiator or photosensitizer of thefollowing formula (I) for use in a photopolymerizable composition:

wherein R₁ represents a monovalent, linear, branched or cyclic,aliphatic hydrocarbon group having 1 to 20 carbon atoms, which isoptionally substituted with one or more substituents selected from —Cl,—Br, —OH, ═O, —NH—CO—OR₂, —NH—CO—R₂ or free-radically or ionicallypolymerizable groups, wherein each R₂ radical is independently selectedfrom —H or C₁₋₆ alkyl; wherein n is >1; Z represents —OR₄—; Y represents—OR₅—; X represents —H or —OH; and R₄ and R₅ each independentlyrepresent a bivalent hydrocarbon group for which otherwise the sameoptions as mentioned for R₁ apply; wherein the polymerizable moieties asoptional substituents of R₁ are selected from polymerizable double ortriple bonds as well as lactam, lactone and epoxide moieties, which aresubjectable to ring-opening polymerization; and wherein any two of R₁ toR₅ may be optionally linked to one another to form a ring or a dimer. 2.The photoinitiator or photosensitizer according to claim 1, wherein, informula (I): a) R₁ represents a linear, branched or cyclic aliphatichydrocarbon group having 1 to 20 carbon atoms, in which one or morecarbon atoms may have been replaced by oxygen atoms and which isoptionally substituted with one or more substituents selected from —Cl,—Br, —OH and —SH; and/or n is >1 and is selected from the range from 2to 100 or from 2 to 50 or from 2 to 20, Z represents —OR₄—, Y represents—OR₅—, and X represents —OH.
 3. The use according to claim 1, whereinthe compound of formula (I) is used as a type I or type IIphotoinitiator.
 4. The photoinitiator or photosensitizer according toclaim 1, wherein the compound of formula (I) is used inphotopolymerizable compositions as a photoinitiator in combination withone or more co-initiators.
 5. The photoinitiator or photosensitizeraccording to claim 4, wherein one or more compounds is/are used asco-initiator/s selected from mono- or polyhydric alcohols (—OH), thiols(—SH), amines (—NR—), silanes (═SiH—), germanes (═GeH—), phosphines(—PRR′R″—), ethers (>CH—O—CH<), iodonium (—I⁺—) and sulfonium (═S⁺—)salts and compounds based on derivatives thereof.
 6. The photoinitiatoror photosensitizer according to claim 5, wherein one or more mono- orpolyhydric alcohols selected from sugars, glycerol, thiols, polyethyleneglycol and polypropylene glycol are used as co-initiators.
 7. Thephotoinitiator or photosensitizer according to claim 1, wherein thecompound of formula (I) is used in an amount of 0.1 to 10 parts byweight per 100 parts by weight of polymerizable monomers.
 8. Thephotoinitiator or photosensitizer according to claim 1, wherein thecompound of formula (I) is used for curing acrylate or methacrylatemonomers.
 9. The photoinitiator or photosensitizer according to claim 1,wherein the compound of formula (I) is used as a photoinitiator orphotosensitizer in compositions to be cured inside the human body or incurable compositions from the food sector.
 10. A photopolymerizablecomposition comprising at least one compound of formula (I) as aphotoinitiator or photosensitizer, at least one photopolymerizablemonomer and optionally at least one co-initiator, fillers, solvents,further initiators or sensitizers, plasticizers and/or flow enhancers:

wherein: R₁ represents a monovalent, linear, branched or cyclic,aliphatic hydrocarbon group having 1 to 20 carbon atoms, which isoptionally substituted with one or more substituents selected from —Cl,—Br, —OH, ═O, —NH—CO—OR₂, —NH—CO—R₂ or free-radically or ionicallypolymerizable groups, wherein each R₂ radical is independently selectedfrom —H or C₁₋₆ alkyl; wherein n is >1; Z represents —OR₄—; Y represents—OR₅—; X represents —H or —OH; and R₄ and R₅ each independentlyrepresent a bivalent hydrocarbon group for which otherwise the sameoptions as mentioned for R₁ apply; wherein the polymerizable moieties asoptional substituents of R₁ are selected from polymerizable double ortriple bonds as well as lactam, lactone and epoxide moieties, which aresubjectable to ring-opening polymerization; and wherein any two of R₁ toR₅ may be optionally linked to one another to form a ring or a dimer.11. The photoinitiator or photosensitizer according to claim 1, whereinn is selected from the range from 2 to
 100. 12. The photoinitiator orphotosensitizer according to claim 11, wherein n is selected from therange from 2 to
 50. 13. The photoinitiator or photosensitizer accordingto claim 12, wherein n is selected from the range from 2 to 20.